Bleaching pulp

ABSTRACT

Pulping liquors used in the bleaching of pulps by hydrogen peroxide, and containing catalase-producing bacteria and/or catalase enzyme are treated with tris (hydroxymethyl) phosphine or a tetrakis (hydroxymethyl) phosphonium salt to kill the bacteria and destroy the enzyme.

[0001] This invention relates to bleaching of pulp by hydrogen peroxideand in particular to a method of treating pulping liquors by preventingor reducing the breakdown of peroxide by catalase.

[0002] Catalase is an enzyme that is produced by bacteria commonly foundin pulp and paper mills. By consuming hydrogen peroxide, catalase canlower bleaching efficiency and decrease brightness levels of thefinished paper.

[0003] It is known to kill catalase-producing bacteria by using abiocide such as glutaraldehyde.

[0004] The bactericidal efficacy of glutaraldehyde againstcatalase-producing bacteria present in pulp and water is known from U.S.Pat. No. 5,728,263. To be of use in pulp operations, a biocide mustadditionally be able to destroy the enzyme chemically.

[0005] It has now been found that tris (hydroxymethyl) phosphine and thetetrakis (hydroxymethyl) phosphonium salts (referred to collectivelyherein as THP) are more effective than glutaraldehyde at killingcatalase-producing bacteria.

[0006] It has also been found that THP can be used more efficiently thanglutaraldehyde to chemically destroy catalase as well as to kill thebacteria that produce it.

[0007] The present invention provides a method of treating pulpingliquors for use in the bleaching of pulp by hydrogen peroxide, saidliquors containing catalase and/or catalase-producing bacteria, with abiocide which reduces or destroys said catalase and/or said bacteria,characterised in that said biocide comprises tris (hydroxymethyl)phosphine (THP) or a tetrakis (hydroxymethyl) phosphonium salt (THPsalt).

[0008] Preferably, the THP salt is tetrakis (hydroxymethyl) phosphoniumsulphate (THPS).

[0009] Alternatively, the THP salt may be tetrakis (hydroxymethyl)phosphonium chloride, phosphate, bromide, carbonate, acetate, citrate,formate, lactate or borate.

[0010] The THP or THP salt is preferably added to the pulping liquor ata concentration of from 5 to 1000 ppm, desirably 10 to 200 ppm, moreusually 15 to 100 ppm, especially 20 to 50 ppm. The pH may be from 4 to12, usually 5 to 10, eg: 7 to 9 in an alkaline pulping system, or 5 to 7in an acid pulping system.

[0011] The invention is illustrated by way of the following examples:

EXAMPLE 1

[0012] Experiments were carried out using a synthetic solution ofcatalase.

[0013] The catalase concentration used was ˜3 ppm.

[0014] Solutions were all buffered at pH 8 (the anticipated pH of thestock chest).

[0015] Contact times of 5, 15 and 30 minutes were allowed.

[0016] Experiments were carried out at 20° C. and 45° C.

[0017] Nominal biocide concentrations of 100 ppm and 600 ppm (ai) wereused.

[0018] Initial hydrogen peroxide concentration=0.5% w/w.

[0019] The experiments used a 75% wt on wt solution of tetrakis(hydroxymethyl) phosphonium sulphate, sold commercially under theRegistered Trade Mark TOLCIDE PS75 and a 50% wt on wt solution ofglutaraldehyde for comparison.

[0020] The principle of the experiments carried out was that when asolution containing active levels of the catalase enzyme is added tohydrogen peroxide, effervescence is observed as the reaction below isfollowed:

2H₂O₂+Catalase→O₂+2H₂O

[0021] For the purpose of the experiments solutions of the catalaseenzyme were contacted with either 100 to 600 ppm (ai) of TOLCIDE® PS75or glutaraldehyde for 5, 15 and 30 minute contact times. Thecatalase/biocide solution was then added to a fixed volume of 0.5% w/whydrogen peroxide and allowed to react. The residual concentration ofhydrogen peroxide was quantified using a potassium permanganatetitration and the % hydrogen peroxide remaining taken as a measure ofthe success of catalase destruction.

[0022] The results obtained are tabulated below in Table 1. TABLE 1Concentration of Contact Biocide/Temperature Time % Hydrogen PeroxideRemaining ° C. (minutes) TOLCIDE ® PS75 Glutaraldehyde 600 ppm/45° C. 537 <1 15 56 3 30 100 100 100 ppm/45° C. 5 <1 <1 15 2 <1 30 76 37 600ppm/20° C. 5 22 <1 15 49 25 30 75 60 100 ppm/20° C. 5 <1 <1 15 18 16 3039 25

[0023] In the absence of biocide treatment NO residual hydrogen peroxidewas observed in the presence of catalase at a 3 ppm level.

[0024] The experiments indicate that TOLCIDE® PS75 is superior toglutaraldehyde for catalase destruction.

EXAMPLE 2

[0025] Samples of de-inked pulp and pulper fill water were received fromtwo de-inking plants, samples 1 and 2. Control needs to be maintainedover bacterial populations within these systems. Bacterial build-up inthe re-cycled alkaline water, and contamination of the recycled fibrecause catalase levels to increase. The catalase breaks down peroxide inthe helico pulper and stops the bleaching effect of the peroxide. Italso means that maintenance of residual peroxide, which is required inthe alkaline loop, is not possible.

[0026] Catalase is produced predominantly by general aerobic bacteria(GAB). During respiration, various toxic oxygen derivatives are producedwithin the bacterial cell, because of this, bacteria produce enzymes todestroy these toxic substances. The most common enzyme in this categoryis catalase, which breaks down hydrogen peroxide to oxygen and water.

[0027] As it is GAB which cause the problems of catalase build-up,quantitative suspension tests (QSTs) were carried out to compare theability of THPS and glutaraldehyde to reduce the number of GAB presentin the pulp/water samples provided.

[0028] An initial test was also carried out whereby mixed pulp/watersamples, which had already been exposed to various concentrations of thetest biocides, then had hydrogen peroxide added to them. The peroxidelevels in these samples was monitored over one hour to gain anindication of the levels of catalase present by the rate of breakdown ofhydrogen peroxide.

[0029] Before carrying out any efficacy tests, material from all of thepulp and water samples provided was plated out onto tryptone soya agarplates and incubated at 45° C., ie: plant operating temperature, for 1-2days.

[0030] This was to ensure that the bacterial populations were similarboth in appearance and, in the case of the water samples, in numbers.

[0031] All water samples were found to contain high levels of GAB ie: inthe order of 10⁷ cfu/ml. (cfu=colony forming units).

[0032] It was assumed that the concentration of the pulp samplesprovided was approximately 15%, therefore a combined pulp/water samplewas prepared by diluting sample 1 pulp with sample 2 water at a ratio of1 in 15 (w/w), thus giving a pulp concentration of approximately 1%,which could be handled relatively easily within these tests. Thisdiluted pulp sample was thoroughly mixed and dispersed in 9.0 g amountsinto sterile universal bottles. These were then incubated at 45° C. for1 hour.

[0033] Immediately prior to beginning the test, stock solutions ofTOLCIDE® PS75 and glutaraldehyde were prepared at the followingconcentrations in sterile WHO standard hardness water:

[0034] 500, 1000, 2000 and 3000 ppm product

[0035] At time zero, 1.0 ml of 10 times the final required biocideconcentration was added to 9.0 g of the diluted pulp, so as to give therange:

[0036] 50, 100, 200 and 300 ppm product for PS75 and glutaraldehyde

[0037] To one 9.0 g sample of diluted pulp, 1.0 ml of sterile WHO wateralone was added to act as a control.

[0038] All samples were then incubated at 45° C.

[0039] Total viable counts (TVCs) of surviving GAB were made on eachsample after contact times of 30 minutes, 1 hour and 3 hours. In orderto do this, serial dilutions were prepared from the samples by initiallyadding 1.0 g of sample to 9.0 ml EST biocide inactivating medium, mixingand allowing to stand for at least 5 minutes. Further serial dilutionswere then made by removing 1.0 ml and adding to 9.0 ml sterile Ringerssolution. From each dilution, 0.1 ml was spread onto tryptone soya agarplates which were inverted and incubated at 45° C. for 2 days prior toenumeration of colonies.

[0040] The above procedure for QST was repeated using pulp and waterfrom sample 2. In this second QST, two additional samples were includedin which 200 ppm product of each biocide was tested. To prepare thesesamples, to 9.09 of chopped pulp, 1.0 ml of 10 volume H₂O₂ (equating toapproximately 0.3% in the pulp) was added and mixed as thoroughly aspossible. 2.0 g of this pulp was then added to 28 g of water sample 2and thoroughly mixed. This pulp dilution was then used for theadditional samples in order to assess the potential effect of H₂O₂ onthe performance of the biocides.

[0041] The results are shown in the following tables 2 to 5:

[0042] Tables 2 and 3 record TVCs in colony forming units per ml(cfu/ml) and log reductions for QSTs on diluted pulp prepared fromsamples 1 and 2 respectively.

[0043] Tables 4 and 5 summarise log reductions achieved by both biocidesin samples 1 and 2 respectively. TABLE 2 QST Results comparing TOLCIDE ®PS75 to Glutaraldehyde in Sample 1 Contact Time (Hours) Conc 0.5 1.0 3.0ppm TVC in Log TVC in Log TVC in Log Biocide product cfu/ml Reductioncfu/ml Reduction cfu/ml Reduction Control 0  4.6 × 10⁷ — 6.7 × 10⁷ — 8.0× 10⁷ — TOLCIDE ® PS75 50 1.69 × 10⁷ 0.43 1.11 × 10⁶  1.78 1.5 × 10⁵2.72 100 1.09 × 10⁵ 2.62 1.01 × 10⁴  3.83 9.0 × 10² 4.95 200  2.8 × 10⁵2.21 1.7 × 10³ 4.60 8.0 × 10² 5.00 300  1.0 × 10⁴ 3.66 3.4 × 10³ 4.301.3 × 10³ 4.79 Glutaraldehyde 50  4.5 × 10⁷ 0.01 2.99 × 10⁷  0.35 3.14 ×10⁶  1.40 100 1.09 × 10⁷ 0.62 1.81 × 10⁶  1.57 1.4 × 10⁵ 2.75 200 1.09 ×10⁶ 1.62 3.6 × 10⁵ 2.27 1.9 × 10⁴ 3.62 300 1.03 × 10⁵ 2.65 4.1 × 10⁴3.22 1.0 × 10³ 4.90

[0044] TABLE 3 QST Results comparing TOLCIDE ® PS75 to Glutaraldehyde inSample 2 Contact Time (Hours) Conc 0.5 1.0 3.0 ppm TVC in Log TVC in LogTVC in Log Biocide product cfu/ml Reduction cfu/ml Reduction cfu/mlReduction Control  0 5.3 × 10⁷ — 2.9 × 10⁷ — 4.3 × 10⁷ — TOLCIDE ® PS75 50 5.1 × 10⁶ 1.01 1.9 × 10⁶ 1.18 7.0 × 10⁵ 1.78 100 4.6 × 10⁵ 2.06 2.0× 10⁵ 2.16 4.3 × 10⁴ 3.00 200 1.3 × 10⁵ 2.61 3.4 × 10⁴ 2.93 2.2 × 10⁴3.29    200P* 1.0 × 10⁵ 2.72 1.6 × 10⁵ 2.26 6.1 × 10⁴ 2.84 300 1.5 × 10⁵2.54 5.8 × 10⁴ 2.70 3.8 × 10⁴ 3.05 Glutaraldehyde  50 4.5 × 10⁷ 0.07 3.5× 10⁷ 0   1.85 × 10⁷  0.36 100 9.1 × 10⁶ 0.76 6.7 × 10⁶ 0.63 4.1 × 10⁶1.02 200 2.83 × 10⁶  1.27 1.21 × 10⁶  1.38 2.7 × 10⁶ 1.20    200P* 3.0 ×10⁶ 1.24 6.4 × 10⁵ 1.65 2.9 × 10⁵ 2.17 300 1.9 × 10⁶ 1.44 1.15 × 10⁶ 1.40 8.1 × 10⁵ 1.72

[0045] TABLE 4 Summary of Log Reductions from QSTs on Sample 1 ContactTime (Hours) Conc 0.5 1.0 3.0 ppm Log Log Log Biocide product ReductionReduction Reduction TOLCIDE ® PS75 50 0.43 1.78 2.72 100 2.62 3.83 4.95200 2.21 4.60 5.00 300 3.66 4.30 4.79 Glutaraldehyde 50 0.01 0.35 1.40100 0.62 1.57 2.75 200 1.62 2.27 3.62 300 2.65 3.22 4.90

[0046] TABLE 5 Summary of Log Reductions from QSTs on Sample 2 ContactTime (Hours) Conc 0.5 1.0 3.0 ppm Log Log Log Biocide product ReductionReduction Reduction TOLCIDE ®  50 1.01 1.18 1.78 PS75 100 2.06 216 3.00200 2.61 2.93 3.29    200P* 2.72 2.26 2.84 300 2.54 2.70 3.05 Glutaral- 50 0.07 0   0.36 dehyde 100 0.76 0.63 1.02 200 1.27 1.38 1.20    200P*1.24 1.65 2.17 300 1.44 1.40 1.72

[0047] Results of these tests suggest that after a 1 hour 15 minutebiocide contact time, THPS has reduced the population of catalaseproducing bacteria more effectively than glutaraldehyde. Results of bothQSTs confirm this.

[0048] By looking at Tables 4 and 5, log reductions achieved by bothbiocides in each QST can be easily compared.

[0049] TOLCIDE® PS75 performs better against the indigenous GAB thandoes glutaraldehyde, particularly at the shorter contact times.

1. A method of treating pulping liquors for use in the bleaching of pulpby hydrogen peroxide, said liquors containing catalase and/orcatalase-producing bacteria, with a biocide which reduces or destroyssaid catalase and/or said bacteria, characterised in that said biocidecomprises tris (hydroxymethyl) phosphine (THP) or a tetrakis(hydroxymethyl) phosphonium salt (THP salt).
 2. A method according toclaim
 1. characterised in that the THP salt is tetrakis (hydroxymethyl)phosphonium sulphate.
 3. A method according to claim 1, characterised inthat the THP salt is tetrakis (hydroxymethyl) phosphonium chloride,phosphate, bromide, carbonate, acetate, citrate, formate, lactate orborate.
 4. A method according to claim 1, 2 or 3, characterised in thatthe THP or THP salt is added to the pulping liquor at a concentration offrom 5 ppm to 1000 ppm.
 5. A method according to claim 4, characterisedin that said concentration is from 10 ppm to 200 ppm.
 6. A methodaccording to claim 4 or 5, characterised in that said concentration isfrom 15 ppm to 100 ppm.
 7. A method according to claim 4, 5 or 6,characterised in that said concentration is from 20 ppm to 50 ppm.
 8. Amethod according to any one of the preceding claims, characterised inthat the pH of the pulping liquor is from 4 to
 12. 9. A method accordingto claim 8, characterised in that the pH is from 5 to
 10. 10. A methodaccording to claim 8 or 9, characterised in that the pH is from 7 to 9in an alkaline pulping system.
 11. A method according to claim 8 or 9,characterised in that the pH is from 5 to 7 in an acid pulping system.